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Wednesday, June 20, 2012

From the sensor to the concert hall

I am very excited to share the following with you! At 9:30 PM on the night of February 3, 2012 two rockfalls occurred into the Halema'uma'u Vent at Kilauea Volcano. These events were reported by HVO and recorded at our station (MENE) in Volcano, HI. This event has now been given new life as the inspiration for a percussion sextet. I am very happy to share with you "Volcano Music Mvt. II" by Jason
Thompson for percussion sextet. Please go to his site and listen to the piece and read his program
notes.UPDATE: unfortunately this piece is no longer available online. The following information and sound files are what was sent to Jason Thompson, and were used to create this wonderful artistic interpretation of a real event.

RAW SOUND:

POST PROCESSED SOUND:

We generally try to clean our tracks up a bit, and the post
processing procedure used here can be found in the following report.

(This report is also available in pdf format on Jason Thompson's website and upon request. Just shoot me an email!)

Origin
time: 03 February 2012 09:30:00 PM HST, 03
February 2012 07:30:00 GMT. All times in narrative are GMT. Description: Two rock falls into the Halemaumau Crater were
reported by the Hawaiian Volcano Observatory (Appendix A) on the night of 02
February 2012 (HST). Associated infrasonic signals were recorded at the local
network.IS Array: UH ISLA - MENE

Data
Quality: Collapses occurred in the quiet of
the night, with very little wind and surf noise.

Method Summary: The
initial signal characterization used automated analysis tools implemented at ISLA.
These include array processing and spectral plots. Further characterization was
completed with the aid of logarithmic scale spectrograms.

1.
Instrumentation

The MENE infrasound array has a
24-bit Reftek 130 digitizer recording four Chaparral 2.2s and one Chaparral 50
infrasonic microphones at 40 samples/second. The Chaparral 2.2s are located at elements
1-4 and have a flat response down to 0.1Hz, and the Chaparral 50 is collated
with MENE1 at element 5 and has a flat response down to 0.02Hz. Array
processing returns the direction of arrival of infrasonic signals as a function
of frequency, which allows a clear azimuthal and spectral separation of source
regions.

2.Chronology

The sequence of events for the Signals of Interest
(SOI1 and SOI2) can be seen in Figure 1, and are described below. Figure 2
shows the signals recorded by the array spanning the times for both signals of
interest. Figures 3 and 4 are
logarithmic spectrograms, and figures 6 and 7 show a four minute window around
the signals of interest.

1)Between 1800 and 2200 GMT of 2 Feb 2012 we have a partition of energy
between the Pu’u O’o Crater complex and the Halemaumau vent, with a clear
frequency and azimuth separation.

2)From around 2100 of 2 Feb 2012 to around 0315 of 3 Feb 2012 we see the
emergence of several harmonics above 1 Hz. Array detections are predominantly
from Halemaumau.

3)From 0315 GMT to ~0615 there is a drop in the overtones, ~1.5 Hz
signal stays, there is temporary drop in the total number of Halemauamau
detections, then the ~1.5 Hz signal strengthens and detections pick back up

4)After ~0615 GMT even the ~1.5Hz signal shuts off, and there is a transition
to deeper frequencies that lasts around 10 minutes. The soundscape becomes
ominously quiet above 1Hz, while there is an increase in observed detections
from the Pu’u O’o azimuth.

5)First collapse signal is at 07:29:20, and the second at 10:06:15. Both
of these signals are broadband and extend down to .02Hz (Figure 3), there is a
change in harmonic structure, with harmonics present below the ~1.5 Hz band and
above the microbarom. Pu’u O’o detections continue and are joined by
Halemaumau.

6)There is a small event at 1500 which may be seismic or volcanic in
origin.

7)After 1600 to the end of the study period, Halemaumau quiets down
while Pu’u O’o resumes radiation, but drops to a lower frequency.

3. Sound
Editing

I select the 24h time period of 3 February,
with Halemaumau as the initial prevailing volcanic source. We preserve all
features in the original data by assigning it different sample rate of 8 kHz
and 44kHz, essentially speeding up the raw data by a factor of 200 and 1102.5
times the original time. We use channel MENE5 because of its better performance
down to 0.02 Hz (50 second periods). The resulting waveforms are shown in
Figure 7.

A time compression of 200x maps 0.02 Hz to 4
Hz, which is below the response of most commercial speakers. It will map the 0.4 - 5 Hz volcanic bandpass
typical of Kilauea to 80 - 1000 Hz. For this event, there are very interesting
features beyond this bandpass that we would like to preserve.

A time compression of 1102.5 maps 0.02 Hz into
22 Hz, which good subwoofers can reproduce. It also maps 5 Hz to 5.512 kHz, well within
the range of most speakers.

Keeps only the volcanic signal and removed
ocean and wind noise in the low register and aircraft noise in the high
registers.

3.5. Now that I’ve deconstructed the sound,
pick a dull noise segment and deconvolve it from the original esound_vie120203_8khz.wav.
Rescale, clean up, save as esound_vie120203_8khz_decon.wav. Remove aircraft by
lowpass filtering below 1500 Hz, and save as esound_vie120203_8khz_decon_lp.wav

Figure 1. Results from the automatic processing. Top panel
is a linear-frequency spectrogram of MENE1, and lower panel is the array
processing detections using PMCC3. Events 1-4 are illustrated with colored
boxes. (1) Green: detections from Halemaumau, harmonics above 1Hz. (2) Red:
decrease in harmonics above 1Hz besides the ~1.5Hz band and initial decrease in
Halemaumau detections with a pickup of Pu’u O’o detections. (3) Yellow: drop in
~1.5Hz band and VLP/ULP event over a
10minute span, Pu’u O’o detections and break in Halemaumau detections. (4) Blue:
SOI-1 with associated coda, and harmonics, SOI-2 and stronger coda in both low
and high frequency, detections from Pu’u O’o and Halemaumau.

Figure 2. Time series around the two signals of interest.
Note the difference between the Chaparral 2.2s (1-4) and the Chaparral 50 (5).
Next figures use only MENE1 and MENE5.

Figure 3. Log-frequency spectrogram from MENE5 over .02Hz to
10hz. Note that SOI-1 and SOI-2 extend down to the lower bound at .02Hz. Also
note the changes in harmonic structures over the time period.

Figure 4. Log-frequency spectrogram from MENE1 over .1 to
10Hz. Also note the change of harmonic structures.

Figure 5. Four
minutes surrounding the first signal of interest with initial downward
deflection and longer low-frequency coda.

Figure 6. Four minutes surrounding the second signal of
interest with a compressional onset and a higher frequency coda.

Past 24 hours at Kilauea summit: The summit tilt network continued
to record DI deflation punctuated with two abrupt positive offsets due
to large rockfalls from the vent rim (Halema`uma`u Crater floor) into the lake
at 9:30 pm and midnight; the first collapse involved a portion of the north rim
while the second took a long sliver of the northeast rim; the first collapse
apparently induced secondary collapses of the inner ledge and ejected hot
spatter onto the nearby portions of the Halema`uma`u Crater floor; the second
collapse deposited a large amount of debris into the northeast side of the lava
lake. Both collapses severely disrupted the lava lake with the second
significantly dropping the level which was slowly recovering lost elevation
this morning. The most recent (preliminary) sulfur dioxide emission rate
measurement was 600 tonnes/day on January 30, 2012.

Seismic tremor levels dropped when a small spattering source appeared on the
north rim of the lake at 8:20 pm last night and remained low with the
two large rockfall seismic signals superimposed. Ten earthquakes were strong
enough to be located beneath Kilauea volcano: one north of and one beneath the
summit caldera, one within the upper east rift zone, and seven on south flank
faults.

Background: The summit lava lake is deep within a ~150 m (500 ft)
diameter cylindrical vent with nearly vertical sides inset within the east wall
and floor of Halema`uma`u Crater. Its level fluctuates from about 70 m to more
than 150 m (out of sight) below the floor of Halema`uma`u Crater. The vent has
been mostly active since opening with a small explosive event on March 19,
2008. Most recently, the lava level of the lake has remained below an inner
ledge (75 m or 250 ft below the floor of Halema`uma`u Crater) and responded to
summit tilt changes with the lake receding during deflation and rising during
inflation.